DEV: Support components under /index paths in themes/plugins (#23876)

Normally, modules defined under `blah/index` can be imported as `blah`. This is also true of Ember resolver lookups - `<MyComponent />` should resolve to the same as `<MyComponent::Index />`. This was working as expected in Discourse core, but we had not implemented the same in our custom resolver logic for themes/plugins.

This commit implements the `/index` fallback, and adds a test for the behaviour.

app/assets/javascripts/discourse-common/addon/resolver.js

@@ -147,7 +147,14 @@ function lookupModuleBySuffix(suffix) {
       }
     });
   }
-  return moduleSuffixTrie.withSuffix(suffix, 1)[0];
+  return (
+    moduleSuffixTrie.withSuffix(suffix, 1)[0] ||
+    moduleSuffixTrie.withSuffix(`${suffix}/index`, 1)[0]
+  );
+}
+
+export function expireModuleTrieCache() {
+  moduleSuffixTrie = null;
 }
 
 export function buildResolver(baseName) {